Alternative transportation fuels have been represented as enablers to reduce toxic emissions in comparison to those generated by conventional fuels. At the same time, tighter emission standards and significant innovation in catalyst formulations and engine controls has led to dramatic improvements in the low emission performance and robustness of gasoline and diesel engine systems. This has certainly reduced the environmental differential between optimized conventional and alternative fuel vehicle systems. However, many technical challenges remain to make the conventionally fueled internal combustion engine a nearly zero emission system having the efficiency necessary to make the vehicle commercially viable.
Alternative fuels cover a wide spectrum of potential environmental benefits, ranging from incremental toxic and carbon dioxide (CO2) emission improvements (reformulated gasoline, alcohols, etc.) to significant toxic and CO2 emission improvements (natural gas, etc.). Hydrogen is clearly the ultimate environmental fuel, with potential as a nearly emission free internal combustion engine fuel (including CO2 if it comes from a non-fossil source).
The automotive industry has made very significant progress in reducing automotive emissions. This has resulted in some added cost and complexity of engine management systems, yet those costs are offset by other advantages of computer controls: increased power density, fuel efficiency, drivability, reliability and real-time diagnostics.
Future initiatives to require zero emission vehicles appear to be taking us into a new regulatory paradigm where asymptotically smaller environmental benefits come at a very large incremental cost. Yet, even an “ultra low emission” certified vehicle can emit high emissions in limited extreme ambient and operating conditions or with failed or degraded components.
One approach to addressing the issue of emissions is the employment of fuel cells. A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. A fuel cell generally consists of two electrodes positioned on opposite sides of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat. There are several types of fuel cells, including proton exchange membrane (PEM) fuel cells and solid oxide fuel cells (SOFC).
In a SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to the external circuit, and producing mostly water and carbon dioxide that are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
Using conventional fuels (i.e., gasoline, diesel) within a fuel cell damages the fuel cell from the deposition of carbon (or soot). Therefore, typical fuel sources for fuel cells are reformates, i.e., a form of purified hydrogen produced from a hydrocarbon fuel processed in a reformer. Three types of reformer technologies are typically employed (steam reformers, dry reformers, and partial oxidation reformers) to convert hydrocarbon fuel (methane, propane, natural gas, gasoline, ete) to hydrogen using water, carbon dioxide, and oxygen, respectively, with byproducts including carbon dioxide and carbon monoxide, accordingly. These reformers operate at high temperatures (e.g., about 800° C. or greater). Under steam reforming, an alcohol, such as methanol or ethanol, or a hydrocarbon, such as methane, gasoline or propane, is reacted with steam over a catalyst. Steam reforming requires elevated temperatures and produces primarily hydrogen and carbon dioxide. Some trace quantities of unreacted reactants and trace quantities of byproducts such as carbon monoxide also result from steam reforming.
Reforming of a fuel generally involves the conversion of a hydrocarbon fuel into separate components, such as hydrogen, carbon monoxide, and carbon dioxide. Generally, lighter hydrocarbons such as methane, ethane, and gasoline more easily reformed. Heavier hydrocarbon fuels, such as diesel, are inherently more difficult to reform because of its tendency to partially decompose prior to full vaporization. Thus, the amount of hydrogen or carbon monoxide recoverable is less than lighter hydrocarbon fuels. Also, diesel fuel is high in sulfur content, which may poison a fuel cell or result in pollution due to the production of sulfur oxides.